This invention relates to a cold trap and more particularly to a cold trap assembly for a liquid metal that has an entrained gas that must be removed. While this invention has particular utility in a nuclear reactor cold trap assembly for the liquid metal coolant to remove entrained cover gases, the cold trap of this invention has utility in many types of apparatus and processes, whether they be chemical processes, conventional heat exchangers, or fossil fuel power plants, just to name a few.
Basic components of a cold trap assembly for liquid metals are the baffle assembly or crystallization element upon which the impurities in the liquid metal precipitate or nucleate, a container for enclosing the crystallization element, a cooling system (gas or liquid) for cooling the liquid metal in the container, and the associated piping, structural supports and necessary control instrumentation. Generally a cold trap assembly also includes a regenerative heat exchanger, so as to minimize system heat losses and total heat removal requirements of the cold trap assembly. As described herein, the cold trap assembly will be called a cold trap unless otherwise specified.
During the operation of liquid metal loop systems used for heat transport in nuclear reactors, impurities are formed by reactions in and with the liquid metal coolant, and these impurities become entrained or dissolved in the liquid metal. These impurities may be transported around the loop system to produce plugging, corrosion, confuse instrumentation, or other deleterious effects unless trapped or filtered out. Cold traps are part of a liquid metal purification system and are used to remove dissolved impurities from the liquid metal by cooling part of the liquid metal to a temperature below the saturation temperature of the impurity. Cold trapping as a method of removing impurities in a liquid metal is based upon solubility-temperature relationships, with the solubility high at high temperatures and low at low temperatures. This cooling occurs in a temperature environment provided with nucleation sites for impurity formation and retention, and thus there is a cleaning or purifying of the liquid metal. Cold traps are designed to produce precipitation or nucleation of the impurities in a known location in a controlled manner and may operate at the lowest temperature of any component in the liquid metal cooling system.
A description of a cold trap appears on pages 138-139 of Fast Reactor Technology: Plant Design edited by John G. Yevick, The M.I.T. Press (1966). Examples of patents on cold traps are U.S. Pat. No. 3,554,374 filed by Roy C. Blair et al., on July 2, 1968 and U.S. Pat. No. 3,618,770 filed by Lawrence E. Pohl et al., on Feb. 4, 1970, each assigned to the assignee of the present invention. As shown in U.S. Pat. No. 3,618,770, which is herein incorporated by reference, a cold trap assembly directs a portion of the bulk stream of liquid metal coolant in a nuclear reactor cooling system to be by-passed through a heat economizer or regenerative heat exchanger and then channeled into a cold trap container where heat rejection takes place. As the temperature of the liquid metal is reduced, the impurities in the coolant nucleate or precipitate leaving a purified liquid metal. In a typical prior art cold trap, the flow is based on the idea that the liquid metal must reside for a short time, as short as 5 minutes, in the cold trap to effectively remove the impurities. For large liquid metal coolant systems, size of the cold trap assembly and flow rates through the cold trap have been based on a residence time greater than the 5 minute residence time in the typical cold trap, and the entire inventory of liquid metal is to be turned over or flow through the cold trap within the range of 12 to 36 hours. Some cold traps for nuclear reactors or large experimental facilities are designed for flow rates of 100 gallons per minute, but generally the maximum flow for an individual cold trap is 60 to 80 gallons per minute for reasons of economy in heat rejection and reliability of the components. Small liquid metal facilities may have cold traps designed for as low as 0.1 gallon per minute. Some cold traps have been demonstrated to show reliable performance at trapping temperatures as low as 230.degree.F, resulting in oxygen levels of 0.1 to 1.5 parts per million. Generally cold traps using a gas as the coolant for cooling the liquid metal in the container will have flow rates of less than 20 gallons per minute, since the heat transfer coefficient of gases are less than that for liquids. The number of cold trap assemblies attached to liquid metal system will range from one to as many as are needed consistent with the economics of the system.
While sodium is one of the most common liquid metal coolants used in nuclear reactors, other well known liquid metals that could be used as coolants are: lead, lithium, mercury, potassium and sodium-potassium alloys. Hydrogen and oxygen are the major soluble impurities in a sodium system, but some of the additional impurities are other interstitials (carbon and nitrogen), or tritium and zenon and other corrosion products.
In the cooling loops using a liquid metal for heat transport, a temperature gradient will be established between the "hot leg" portions and "cold leg" portions. When the liquid metal used as the coolant in the loops is sodium, as in the case of liquid metal fast breeder reactors (LMFBR's), a cover gas must be used for occupying all gas spaces contiguous with the surfaces of the liquid sodium to prevent reactions, impurity formations and adverse effects, such as corrosion. The cover gas is usually an inert gas such as argon or helium. During circulation of the liquid metal coolant through the loops, some of the cover gas will become entrained or dissolved in the liquid metal. Solubility of cover gases in liquid metal is dependent upon the temperature with the solubility high at high temperatures and low at low temperatures. During liquid metal purification in a cold trap where impurities are nucleated or precipitated, as the temperature of the liquid metal decreases, a major portion of the various dissolved cover gases reach the temperature of saturation solubility and thus these gases are released into the cold trap assembly. The "released" cover gases are absorbed by or dissolved in the liquid metal in the "hot leg" portions of the cooling system at the liquid metal-cover gas interface. The efficiency of the removal of impurities by a cold trap assembly will be reduced as a result of the "released" cover gases occupying a part of the cold trap volume. The reduced volume available for the liquid metal to occupy will make less of the surface area available for heat transfer and precipitation and this reduces the efficiency of the cold trap and decreases the residence time for sodium in the cold trap. In addition, a sudden release of accumulated cover gas from the cold trap into the main loop of the liquid metal coolant system can cause severe flow perturbations, which in many cases have heretofore caused loop dumps, or severe perturbations during reactor operation so as to effect the operation of a nuclear reactor.
Prior art devices have attempted to deal with this problem of cover gas accumulation by installing an accumulator in the cold trap loop downstream of the cold trap which collects all the released cover gas. However, the liquid metal level in such an accumulator must be constantly monitored and cover gas venting must be performed at regular intervals. Insofar as these problems of cover gas accumulation are not confined to small sodium loops, it can be readily expected that larger cover gas bubbles in the sodium of a large liquid metal fast breeder reactor (LMFBR) could cause severe problems in the core region by blanketing the fuel elements.